Micromechanical devices are small structures typically fabricated on a semiconductor wafer using techniques such as optical lithography, doping, metal sputtering, oxide deposition, and plasma etching which have been developed for the fabrication of integrated circuits.
Micromirror devices are a type of micromechanical device. Other types of micromechanical devices include accelerometers, pressure and flow sensors, gears and motors. While some micromechanical devices, such as pressure sensors, flow sensors, and micromirrors have found commercial success, other types have not yet been commercially viable.
Digital micromirror devices are primarily used in optical display systems. In display systems, the micromirror is a light modulator that uses digital image data to modulate a beam of light by selectively reflecting portions of the beam of light to a display screen. While analog modes of operation are possible, micromirrors typically operate in a digital bistable mode of operation and as such are the core of the first true digital full-color image projection systems.
Micromirrors have evolved rapidly over the past ten to fifteen years. Early devices used a deformable reflective membrane which, when electrostatically attracted to an underlying address electrode, dimpled toward the address electrode. Schlieren optics illuminate the membrane and create an image from the light scattered by the dimpled portions of the membrane. Schlieren systems enabled the membrane devices to form images, but the images formed were very dim and had low contrast ratios, making them unsuitable for most image display applications.
Later micromirror devices used flaps or diving board-shaped cantilever beams of silicon or aluminum, coupled with dark-field optics to create images having improved contrast ratios. Flap and cantilever beam devices typically used a single metal layer to form the top reflective layer of the device. This single metal layer tended to deform over a large region, however, which scattered light impinging on the deformed portion. Torsion beam devices use a thin metal layer to form a torsion beam, which is referred to as a hinge, and a thicker metal layer to form a rigid member, or beam, typically having a mirror-like surface: concentrating the deformation on a relatively small portion of the micromirror surface. The rigid mirror remains flat while the hinges deform, minimizing the amount of light scattered by the device and improving the contrast ratio of the device.
Recent micromirror configurations, called hidden-hinge designs, further improve the image contrast ratio by fabricating the mirror on a pedestal above the torsion beams. The elevated mirror covers the torsion beams, torsion beam supports, and a rigid yoke connecting the torsion beams and mirror support, further improving the contrast ratio of images produced by the device. Additional contrast improvements have been provided by various coatings applied to the substrate of other areas underneath the mirrors.
Yet another recent micromirror configuration, called a spring-ring design, provide an intermediate resilient member to land the micromirror on. The resilient member is deformed when the mirror lands and stores this potential energy until the electrostatic force deflecting the mirror is removed. When the mirror is released, the resilient member springs the mirror back toward its neutral position.
All previous torsion beam micromirror designs have used a memory cell to create an electric field between the mirror and an address electrode on each side of the torsion hinge. The voltage differential between the mirror and one of the address electrodes is greater than between the mirror and the other address electrode—causing a greater electrostatic attraction to occur on the side having the greatest voltage differential. This unbalanced attractive force deflects the mirror to the side having the greatest attractive force.
Using the memory cell to generate a voltage has an undesired side effect on the design of the micromirror circuitry. The difference between the two address electrodes must fairly substantial to enable reliable control of the mirror position. Advances in CMOS semiconductor processes, however, are intended to reduce the feature size of the circuitry and reduce the power consumption of the circuitry. Both of these advances tend to result in lower voltage operation of the CMOS circuitry. Thus, it is becoming increasingly difficult to use standard modern CMOS processes to fabricate a memory cell that will reliably drive the address voltages of a micromirror device at a level sufficient to control the mirror position.
What is needed is a method and system for positioning the micromirror that provides reliable mirror positioning along with an excellent lifetime reliability, yet is able to be manufactured using both the fabrication processes in use today as well as advanced fabrication techniques likely to be developed in the immediate future.